Welcome to the Apple Support Communities.
I found the following information that may be helpful:
If a podcast is not downloading, it may be due to the server that hosts the podcast, or because there is an issue with your current Internet connection. Deleting the podcast from Downloads may help solve the issue. To delete a podcast that is downloading or waiting to download, tap Downloads, swipe left or right over the podcast, then tap Delete.
Depending on your device, podcasts can be streamed and downloaded over Wi-Fi or cellular data connections. When using a cellular data connection, there may be a limit on the maximum podcast download size.